The invention is concerned with a phase synchronizing circuit or a phase lock loop that follows the frequency and the phase of its input signal.
The conventional phase synchronizing circuit of the PLL (Phase Lock Loop) type has been widely used, e.g., as in a carrier regeneration circuit that provides a reference signal required in demodulating a phase modulated wave or a frequency tracking circuit that traces an electromagnetic wave.
The phase synchronizing circuit of the PLL type, however, has a deterioration in the phase pull-in characteristic known as the hangup phenomenon, and this makes it difficult to use the circuit in fields that require high speed synchronization.
In the first place, let's show a circuit of the PLL type as an example, and explain the hangup phenomenon.
The basic functional block diagram of a circuit of the PLL type is shown in FIG. 1. In FIG. 1, 1 stands for an input terminal for signal, 2 stands for a voltage controlled oscillator (V.C.O.), 3 stands for a phase comparator, 4 stands for a loop filter, 5 stands for an output terminal for phase synchronized signal. If a coming signal is modulated, its modulating components are assumed to be removed before it is applied to the input terminal 1.
The circuit of FIG. 1 is designed to provide the output signal of voltage controlled oscillator 2 that is synchronized in frequency and phase with an input signal at input terminal 1. That is, the phase comparison between an input signal and the output signal of voltage controlled oscillator 2 is performed at phase comparator 3, and the phase difference obtained is fed back, as a voltage, to voltage controlled oscillator via loop filter 4, so that the frequency and the phase of the output signal of voltage controlled oscillator 2 trace the frequency and the phase of the input signal respectively.
An example of the phase comparison characteristic of phase comparator 3 is shown in FIG. 7(a). This is well known as the sinusoidal characteristic and has been popularly used in practical applications.
In FIG. 7(a), x axis represents the phase difference between the phase of an input signal and that of the output signal of voltage controlled oscillator 2, and y axis represents the output signal (or error voltage) of phase comparator 3.
From FIG. 7(a), it is easily seen that if the phase difference between an input signal and the output signal of voltage controlled oscillator 2 becomes .pi. (or 180.degree.) the corresponding output signal of phase comparator 3 becomes zero, and the oscillating phase of voltage controlled oscillator 2 gets stable leaving the phase difference .pi. unchanged. Therefore, synchronization cannot be established. This condition is a so-called hangup phenomenon. Even when the phase difference is not exactly equal to .pi. but almost equal to .pi., the condition can be considered as a hangup phenomenon in practical application. If the phase difference is not so near to .pi. but in the vicinity of .pi., it takes a long time to establish synchronization as the output of phase comparator is very small. These are drawbacks of the conventional phase synchronizing circuit.
The drawbacks make it difficult to use a phase synchronizing circuit having the phase comparison characteristic of FIG. 7(a) as a carrier regeneration circuit for use in demodulating a time-division multiple-access (TDMA) signal composed of a number of mutually asynchronous bursts. The reason is that the circuit should give a reference signal for each burst in demodulating the TDMA signal, and this requires the circuit's performance to establish synchronization in a quite short time.
Meanwhile, methods are proposed to remove the said drawbacks of the conventional phase synchronizing circuit in a paper titled "Examination of carrier generation circuits for use in synchronized demodulation of TDMA signals." (Trans. of the Institute of Electronics and Communication Engineers of Japan, Vol. 54-B, No. 4, pp. 160/167.)
According to one of the methods, phase comparator 3 in the circuit of the PLL type shown in FIG. 1 has a phase comparison characteristic that gives a saw-tooth waveform against phase difference as shown in FIG. 7(b). The phase comparison characteristic is considered effective in bringing about high speed synchronization converging to stabilize a phase difference point such as 0 or 2.pi., by providing a large output amplitude when a phase difference is in the vicinity of .pi.. Even in this method, however, if an input signal contains some phase jitter or noise, the outputs around the phase difference .pi. get averaged and this causes the saw-tooth phase comparison characteristic of FIG. 7(b) change to an equivalent phase comparison characteristic such as shown in FIG. 7(c). Therefore, time required in establishing synchronization becomes longer than is expected in design.
Another method described in the paper is called a kick-off method. In this method, measurement of an initial phase difference is done when a process for synchronization gets started, and if the measured phase difference is in the vicinity of .pi., a phase shift of .pi. is forced to occur in the output signal of the voltage controlled oscillator to move the initial hangup region (or .pi.-neighborhood) to a stable region (e.g., 0-neighborhood). The method, however, has a drawback that it is of no use if the beginning time point of a synchronizing process fails to be detected. In addition, with an input signal containing some noise, misjudging problems concerning hangup phenomena may arise, e.g., such cases as overlooking an existing hangup phenomenon or triggering a phase shift of .pi. when no hangup phenomenon really exists. Therefore, these methods do not give a perfect solution for the hangup phenomenon.